In electronic systems for controlling the brake force (ABS) or for influencing/checking the driving stability (ESP, TCS), it is common to employ a unit for controlling/regulating and information processing that is connected to sensors and actuators of most various types, such as wheel speed sensors, filling level sensors, electromagnetic hydraulic valves, relays, and like components.
When the motor vehicle is switched off, e.g. when being parked, the electronic system will switch off immediately or, optionally, a certain time after switch-off of the vehicle; however, while the electronic system is deactivated, the vehicle condition will generally change. For example, the pressure in a hydraulic reservoir is likely to drop in the inactive state of the vehicle. Therefore, the current state of the vehicle is unknown to the electronic system shortly after the activation of the electronic system. Consequently, it is impossible to poll the vehicle condition when the controller is deactivated without having activated the controller before].
It is even impossible to overcome this problem by providing non-volatile memories such as Flash ROMs, instead of volatile memories such as dynamic or static RAMS, to store data about the vehicle state, because the data about the vehicle state cannot be updated when the electronic system is switched off.
If, for example in a motor vehicle equipped with an electrohydraulic brake (EHB), the objective is to monitor the filling level of the hydraulic accumulator that pertains to the EHB, this is not easily accomplished with prior art electronic controllers that switch off constantly. Especially long-term effects, such as the reduction of the pressure in the said hydraulic accumulator due to a slight leakage of the pressure reservoir or the supply lines, are impossible to monitor. When the pressure of the hydraulic reservoir in an EHB brake system drops below a defined value, the pressure is required to be re-increased by means of a pressure increase device (pump) in order that the full brake pressure is available again, if possible, directly after a brake operation is triggered by the driver. When a motor vehicle with EHB has been inactive for a long period of time, the electronic system will only be able to detect a pressure drop in the hydraulic reservoir when the driver starts the vehicle. However, the full pressure will not yet be available in the hydraulic reservoir at this moment. The consequential risk is that the brake system is not ready for operation immediately after release of the parking brake.
Activation of the electronic system over a length of time is, however, disadvantageous, in various respects. Thus, the permanent operation may e.g. cause high power consumption, a permanent heating, or an excessive stress of the electronic components.